In a semiconductor device manufacturing process, when an insulating film formed on a substrate is etched, an etching process of high selectivity is required to prevent a base film from being damaged. For example, when a silicon oxide film formed on a substrate is etched by a dual stress liner technique, high etching selectivity of the silicon oxide film to a silicon nitride film is required to prevent the silicon nitride film as a base from being damaged.
The dual stress liner technique is a technique for applying tensile stress to an N channel type FET (Field Effect Transistor) by covering the N channel type FET with a silicon nitride film and for applying compressive stress to a P channel type FET by covering the P channel type FET with a silicon nitride film (see, e.g., Japanese Patent Application Publication No. 2007-088452)). A drain current of a transistor is increased by applying stress to the transistor. Accordingly, performance of the transistor can be improved.
In the dual stress linear technique, the silicon nitride film to which the tensile stress is applied and the silicon nitride film to which the compressive stress is applied are separately formed. Therefore, (1) a silicon oxide film, (2) a silicon nitride film, and (3) a silicon oxide film are laminated on a substrate in that order. Then, a process of etching (3) the silicon oxide film and (2) the silicon oxide film is required. As for the etching, a dry etching of introducing a processing gas into an airtight processing chamber, turning the processing gas into a plasma, and exposing an insulating film to be etched to the processing gas that has been turned into the plasma is applied. As described above, when (3) the silicon oxide film is etched, it is required to increase selectivity of (3) silicon oxide film to (2) the silicon nitride film. In order to increase the selectivity of (3) the silicon oxide film to (2) the silicon nitride film, a CF-based or a CHF-based etching gas which performs film forming reaction and etching reaction simultaneously is used as the etching gas. Therefore, the etching is performed while balancing deposition of a CF-based deposit and etching.
However, in the case of using the CF-based or the CHF-based etching gas, a CF-based deposit remains on a surface of (2) the silicon nitride film upon completion of etching of (3) the silicon oxide film (completion of over etching). If the CF-based deposit remains, the deposit serves as an etching mask, and the etching of (2) the silicon nitride film as a base film does not occur locally.
To solve this problem, it is considered to use an ashing technique for removing the deposit by generating an oxygen plasma and reacting the oxygen plasma with the deposit upon completion of the over etching of the silicon oxide film.
However, when the oxygen plasma used in the ashing has a high energy, the surface of (2) the silicon nitride film is oxidized by the oxygen plasma, and a silicon oxide film is formed on the surface of (2) the silicon nitride film. If the silicon oxide film is formed, the etching of (2) the silicon nitride film in a next process is not carried out.
Further, in an etching process for forming an insulating film on a sidewall of a gate electrode, it is required to prevent a substrate from being damaged (recessed) by an oxygen plasma in order to manufacture a device in accordance with a design.